1. Field of the Invention
The present invention relates to solvent-based pigmented coating compositions containing gloss reducing agents and a process for preparing such compositions.
2. Description of Related Art
Thermosettable coating formulations, particularly polyester or diester-based coating compositions, are often the materials of choice for application to various substrates, particularly metal substrates, as a paint or a protective coating. Such coatings can be formulated to provide a good balance of properties such as hardness, flexibility, solvent resistance, corrosion resistance, weatherability and gloss. The enhancement of these properties depends on many factors including type, molecular weight, monomer composition, and glass transition temperature (Tg) of the resin; type and amount of the crosslinker; curing conditions; curing catalysts; pigments; fillers and additives. Variations of these parameters can be used to create a wide range of differences in film properties to fit requirements for a number of diverse applications. However, it is not always possible to optimize all of the desirable properties simultaneously.
One of the properties in need of control in such compositions is the degree of finish gloss of the cured coating. Regulation of gloss is important in the development of solvent-based pigmented coatings. Depending on the ultimate application, cured coatings may desirably have a high gloss (&gt;70), intermediate gloss (50-60), semi-gloss (20-40) or low gloss (&lt;15), measured as % gloss at 60.degree. in accordance with ASTM D-523.
One known method for controlling gloss in high gloss, pigmented coating compositions is to include a quantity of so called "flatting agents" into the coating formulation. These agents are finely divided particulate materials of irregular shape which tend to dull the surface appearance of the cured coating by dispersing incident light rays, and include materials such as silica, barytes, diatomaceous earth and heavy metal soaps.
Some of the disadvantages of using these materials to control gloss is that they must be added to the coating formulation as very fine dispersions in suitable organic solvents, which can contribute to a higher-than-desirable content of volatile solvents in the coating formulation. They can also affect the degree of crosslinking which can be achieved when the applied coating is cured, particularly in coating formulations containing relatively high pigment loadings.
Gloss regulation also appears to be more difficult for higher pigment loadings.
Gloss reduction in powdered, epoxy-based coatings has also been achieved by including into the formulation a minor amount of a copolymer of ethylene and acrylic acid, as disclosed in U.S. Pat. No. 4,419,495.
There exists, however, a continuing need to develop gloss control agents which are effectively used in solvent-based, pigmented, resin-based coating formulations and which minimally affect the physical and chemical properties of the cured coating, especially for coatings formulations with high pigment to binder ratios.